In recent years, color liquid crystal display devices have been used widely in various fields, as monitors for portable notebook-type personal computers, ordinary personal computers, and the like or as display sections in liquid crystal televisions, video-integrated liquid crystal televisions, cellular phones, PDAs, and the like. In accordance with increase of data processing volumes, diversification of needs, compatibility with multimedia, and the like, developments have been eagerly made to achieve liquid crystal display devices having a larger screen size and higher resolution.
A liquid crystal display device is basically constituted by a backlight part and a liquid crystal display part. The backlight part is of a directly under light system in which a primary light source is provided directly under the liquid crystal display part or of an edge light system in which a primary light source is provided so as to face a side end face of a light guide. From the viewpoint of miniaturization of the liquid crystal display device, the edge light system is used frequently.
In recent years, there has been a display device having a relatively small screen size and a relatively narrow range of viewable direction, e.g., a liquid crystal display device used as a display part of a cellular phone. This display device uses such a backlight part of the edge light system that narrows as much as possible the spread angle of light flux emitted from the screen to emit light concentrated on a required angular range, in order to utilize effectively the amount of light generated from a primary light source, from the viewpoint of reduction in power consumption.
JP(A)-2001-143515 has proposed use of a prism sheet in a light source device which is used in a display device having a limited range of viewable direction and emits light concentrated on a relatively narrow range to raise utilization efficiency of the amount of light emitted from a primary light source and to reduce power consumption. The prism sheet has prism formed surfaces on both sides and is disposed adjacent to a light emitting face of a light guide. In this double-sided prism sheet, plural elongated prisms extending in parallel to each other are formed on each of a light incoming surface as one prism formed surface and a light outgoing surface as another prism formed surface. The directions of the elongated prisms are arranged to coincide with each other between the light incoming surface and the light outgoing surface, and the elongated prisms formed on both sides are provided at corresponding positions. As a result of this, light which is emitted from the light emitting face of the light guide so as to have a distribution within an appropriate angular range with a peak in a direction inclined to the light emitting face, is let enter from one prism face of the light incoming surface of the prism sheet and is internally reflected by another prism face thereof. Further, the light is subjected to refraction by the elongated prisms on the light outgoing surface, to concentrate and emit the light in required directions within a relatively narrow range.
According to this kind of light source device, concentrated emission within a narrow angular range is possible. However, the plural elongated prisms formed in parallel to each other on both sides of the prism sheet used as an optical deflector element need to be arranged such that the directions of elongated prisms are matched with each other and the elongated prisms are provided at positions corresponding to each other between the light incoming surface and light outgoing surface. Molding of these parts is therefore complicated.
On the other side, JP(A)-10-254371 has proposed improvement in luminance in the normal direction of a prism sheet. The improvement is achieved by setting the inclination angle of one prism face to 4.7 to 5.7 degrees with respect to the normal direction of the prism sheet, as well as setting the inclination angle of another prism face to 34.2 to 35 degrees with respect to the normal direction of the prism sheet, wherein the one prism face and the another prism face constitute an elongated prism of the prism sheet. However, in this kind of prism sheet in which the inclination angle of one prism face of elongated prism to the normal direction of the prism sheet is thus extremely small, the apex angle of the elongated prism is as small as 40 degrees or so. Consequently, such a fine prism shape is difficult to mold, and black stripes are observed on the basis of “deformation” at top edges of the elongated prisms if such a prism is used to construct a light source device. Thus, there has been a problem in quality level as a light source.
In addition, for the purpose of improving resistance against scratch damages at top edges of elongated prisms without causing a great deterioration in luminance, JP(A)-2001-343507 has proposed formation of flat parts parallel to sheet planes at top edges of the elongated prisms of the prism sheet. If a prism sheet in which flat parts parallel to sheet planes are formed at top edges of the elongated prisms is used to construct a light source device, occurrence of black stripes caused by “deformation” of top edges of the elongated prisms can be suppressed to some extent. However, there is a problem that the contact area between a light guide and the prism sheet increases to thereby cause sticking followed by occurrence of a Moire pattern or deterioration in luminance for a light source device.